1. Field of the Invention
The present invention relates to an optical pickup that is capable of changing a phase distribution of light emitted from a plurality of light sources, and performs at least one of recording, reproduction and erasure of data in or from an optical recording medium, and an optical data processing apparatus including the optical pickup.
2. Description of the Related Art
Optical recording media, for example, a CD (Compact Disk) having a capacity of 0.65 GB, and a DVD (Digital Versatile Disk) having a capacity of 4.7 GB, are widely used for storing video and audio information, and data processed in a computer.
There are various ways to increase the recording density of an optical recording medium. For example, in an optical pickup, which reads data from or writes data in the optical recording medium, the numerical aperture (NA) of the object lens may be increased, or alternatively, the wavelength of light from a light source may be shortened, thereby reducing the size of a beam spot formed on the optical recording medium by a light beam condensed by the object lens.
For this reason, according to the current standard, in an optical pickup used for a CD-type optical recording medium, for example, the NA of the object lens is specified to be 0.50, and the wavelength of the light beam for data operations on the CD-type optical recording medium is specified to be 780 nm; in an optical pickup used for a DVD-type optical recording medium, for example, the NA of the object lens is specified to be 0.65, and the wavelength of the light beam for operations is specified to be 660 nm. In order to further increase the recording density of the optical recording medium, a new standard is required that enables a numerical aperture greater than 0.65, and a wavelength of the light beam shorter than 660 nm.
Such a new standard that enables usage of a larger numerical aperture and a shorter wavelength of the light beam has been proposed recently. On the other hand, a great number of CDs and DVDs conforming to the old standards are held by users. Thus, it is desirable that an optical information processing apparatus be able to handle both the optical recording media conforming to the new standard and those conforming to the old standards.
A simple way of achieving such compatibility is to include both a conventional optical pickup for the optical recording media conforming to the old standards and an optical pickup for the optical recording media conforming to the new standard in the same optical information processing apparatus. However, in this case, it is difficult to reduce the size and cost of the optical information processing apparatus.
FIG. 1 is a block diagram showing a configuration of an optical information processing apparatus able to be made compact.
In order to obtain compatibility between a CD-type optical recording medium and a DVD-type optical recording medium, and further a blue-light optical recording medium (an optical recording medium which is operated by a light beam in the blue wavelength region and thus has a lager storage capacity), the configuration shown in FIG. 1 may be used, which includes a light source 100 emitting blue light for operating the blue-light optical recording medium, a light source 200 for the DVD-type optical recording medium, a light source 300 for the CD-type optical recording medium, and one object lens for condensing light from the light sources 100, 200, and 300 on corresponding optical recording media.
However, there exist problems in the optical information processing apparatus shown in FIG. 1, which uses only one object lens to condense light of different wavelengths onto CD-type, DVD-type, and blue-light optical recording media, which conform to different standards.
First, different optical recording media, that is, optical recording media conforming to different standards, require different numerical apertures of the object lens. Therefore, with the apparatus shown in FIG. 1, it is necessary to change the numerical aperture of the object lens depending on the optical recording medium on or from which data are recorded or reproduced.
For example, an NA switching element for realizing compatibility between a DVD-type optical recording medium and a CD-type optical recording medium is disclosed in Japanese Patent Gazette No. 3240846 (referred to as “reference 1” hereinafter), Japanese Patent Gazette No. 2713257 (referred to as “reference 2” hereinafter), Japanese Patent Gazette No. 2725653 (referred to as “reference 3” hereinafter), and Japanese Utility Model Gazette No. 3036314 (referred to as “reference 4” hereinafter).
An NA switching element for realizing compatibility between a blue-light large capacity optical recording medium and a DVD-type optical recording medium is disclosed in Japanese Laid-Open Patent Application No. 2001-216676 (referred to as “reference 5” hereinafter).
A three-stage NA switching element for realizing compatibility between a blue-light large capacity optical recording medium, a DVD-type optical recording medium and a CD-type optical recording medium is disclosed in Japanese Laid-Open Patent Application No. 2000-187870 (referred to as “reference 6” hereinafter), and Japanese Laid-Open Patent Application No. 2003-67972 (referred to as “reference 7” hereinafter).
For example, consider an optical pickup that is able to handle the blue-light optical recording medium, which is irradiated by a light beam having a light wavelength (λ1) equaling 407 nm, corresponds to a numerical aperture (NA(λ1)) equaling 0.67, and has a substrate thickness (t1) equaling 0.6 mm on the light irradiation side; a DVD-type optical recording medium, which is irradiated by a light beam having a light wavelength (λ2) equaling 660 nm, corresponds to a numerical aperture (NA(λ2)) equaling 0.65, and has a substrate thickness (t2) equaling 0.6 mm on the light irradiation side; and a CD-type optical recording medium, which is irradiated by a light beam having a wavelength (λ3) equaling 780 nm, corresponds to a numerical aperture (NA(λ3)) equaling 0.50, and has a substrate thickness (t3) equaling 1.2 mm on the light irradiation side.
Here, it is assumed that the object lens is designed to produce a minimum spherical aberration when a parallel blue light beam for the blue-light optical recording medium is incident on the object lens (parallel light incidence is referred to as “infinite incidence” below where appropriate). When this object lens is irradiated by the light beam having a wavelength of 660 nm by infinite incidence so as to form a spot on the DVD-type optical recording medium, a spherical aberration occurs (described below with reference to FIG. 8B) due to the difference of the light wavelengths (407 nm and 660 nm).
Similarly, when the object lens is irradiated by the light beam having a wavelength of 780 nm by infinite incidence to form a spot on the CD-type optical recording medium, a spherical aberration occurs, too (described below with reference to FIG. 9B) because of the difference of the light source wavelengths (407 nm and 780 nm).
This problem occurs also for an optical pickup that handles only the DVD-type optical recording medium and the CD-type optical recording medium.
For example, the DVD-type optical recording medium is related to a light wavelength (λ2) equaling 660 nm, a numerical aperture (NA(λ2)) equaling 0.65, and a substrate thickness (t2) equaling 0.6 mm on the light irradiation side, and the CD-type optical recording medium is related to a light wavelength (λ3) equaling 780 nm, a numerical aperture (NA(λ3)) equaling 0.50, and a substrate thickness (t3) equaling 1.2 mm on the light irradiation side.
It is assumed that the object lens is designed to produce a minimum spherical aberration when a parallel light beam having a wavelength of 660 nm for the DVD optical recording medium is incident on the object lens. If this object lens is irradiated by the light beam having a wavelength of 780 nm by infinite incidence to form a spot on the CD-type optical recording medium, again, a spherical aberration occurs due to the difference of the light source wavelengths (660 nm and 780 nm).
The reference 3 and Japanese Laid-Open Patent Application No. 10-334504 (referred to as “reference 8” hereinafter) disclose optical pickups to solve this problem. Specifically, the optical pickup disclosed in reference 3 or reference 8 includes two semiconductor lasers emitting light beams of different wavelengths, and a wavelength-selective phase shifting element. One of the semiconductor lasers emits a light beam having a wavelength of 660 nm for recording or reproducing data in a DVD-type optical recording medium having a substrate thickness of 0.6 mm, and the other semiconductor laser emits a light beam having a wavelength of 780 nm for recording or reproducing data in a CD-type optical recording medium having a substrate thickness of 1.2 mm. The wavelength-selective phase shifting element does not change the phase distribution of the light beam having a wavelength of 660 nm, but does change the phase distribution of the light beam having a wavelength of 780 nm so as to correct the spherical aberration caused by the difference of the substrate thickness.
There is another well known method for solving the aforesaid problems, in which the light beam having a wavelength of 660 nm is incident on the DVD side of the object lens by infinite incidence, and the light beam having a wavelength of 780 nm is incident on the CD side of the object lens by finite incidence (that is, the incident light beam is diverging or focusing), thereby correcting the spherical aberration caused by difference of the substrate thickness.
Japanese Laid-Open Patent Application No. 2000-348366 (referred to as “reference 9” hereinafter) also discloses an invention related to the present field.
Further, based on the aforesaid related art, a method for recording data to or reproducing data from a CD-type optical recording medium, a DVD-type optical recording medium, and a blue-light optical recording medium with one object lens is proposed in ISOM 2001 Conference Abstract, pp 30-31, “Blue/DVD/CD Compatible Optical Head with Three Wavelengths and A Wavelength Selective Filter”, Ryuichi Katayama and Yuichi Komatsu (referred to as “reference 10” hereinafter). Specifically, the optical pickup includes three semiconductor lasers emitting light beams having respective wavelengths of 405 nm, 650 nm, and 780 nm, and a wavelength-selective phase shifting element. The 405 nm light beam is incident, by infinite incidence, on a blue-light optical recording medium having a substrate thickness of 0.1 mm; the light beam having a wavelength of 660 nm is incident, by finite incidence, on a DVD-type optical recording medium having a substrate thickness of 0.6 mm; and the light beam having a wavelength of 780 nm is incident, by finite incidence, on a CD-type optical recording medium having a substrate thickness of 1.2 mm. The wavelength-selective phase shifting element does not change the phase distribution of the 405 nm light beam, but does change the phase distributions of the 660 nm light beam and the light beam having a wavelength of 780 nm to correct the spherical aberrations caused by the differences of the substrate thickness. In other words, this method utilizes two wave-front correction methods at the same time; one is the wavelength-selective phase shifting element, and the other is the finite incidence for the 660 nm light beam and the light beam having a wavelength of 780 nm.
Usually, the aforesaid NA switching element is mounted on an actuator, which moves the object lens in a focusing direction and tracking direction, so that the NA switching element is movable together with the object lens to keep the NA unchanged. In addition, the phase shifting element is also provided to be movable together with the object lens, because a deviation of the phase shifting element relative to an optical axis of the object lens may cause a coma aberration.
On the other hand, it is desirable that the number of parts of the actuator be reduced so as to reduce the weight of the actuator and reduce the number of steps of assembling the actuator. For example, one may attempt to bond the phase shifting element and the NA switching element together to simplify the process of assembling the phase shifting element and the NA switching element onto the actuator. But this does not reduce the weight of the actuator. In addition, one may attempt to use front and back surfaces of a glass substrate as the NA switching element and the phase shifting element, respectively. However, when fabricating a slim device, usually after the process of forming a surface shape on the front side of the glass substrate, the back side of the glass substrate is polished so as to reduce the thickness of the substrate. Therefore, if surface shapes are formed on both the front side and the back side of the glass substrate as the NA switching element and the phase shifting element, a relatively thick glass substrate is required, and after the surface shapes are formed on both sides, the glass substrate cannot be polished. That is, it is difficult to make the substrate thin. Furthermore, it is also difficult to control alignment accuracy of the front and the back sides.
Further, with the methods disclosed in the reference 10 and the reference 7, it is difficult to obtain sufficiently high wave-front performance for both the DVD-type optical recording medium and the CD-type optical recording medium. Generally, for the wave-front having an aberration at the diffraction limit, the Marechal criterion of 0.07 λrms is used as a reference value of the aberration. In a practical optical pickup, however, there exist many kinds of errors, such as a thickness error, a tilt error of the optical recording medium, a defocus error related to position deviation of the object lens relative to the optical recording medium, and so on, and the aberration caused by these errors results in wave-front degradation. Thus, it is desirable that the aberration without contributions from the above errors, that is, the central value of the aberration, be less than or equal to 0.03 λrms.
In the reference 10 and the reference 7, however, the central value of the aberration related to the DVD-type optical recording medium is about 0.05 λrms, which is greater than 0.03 λrms. This large aberration is due to the difficulty in minimizing the aberration related to both the DVD-type optical recording medium and the CD-type optical recording medium with a single optical element. In other words, when designing the phase correction element, one has to adopt only mean values of the theoretical optimal values, which give minimum aberrations for both the DVD-type optical recording medium and the CD-type optical recording medium, and as a result, it is difficult to sufficiently suppress the aberration for both the DVD-type optical recording medium and the CD-type optical recording medium.
In addition, the three-stage NA switching element used for the blue-light optical recording medium, the DVD-type optical recording medium and the CD-type optical recording medium, as disclosed in the reference 6 and the reference 7, has a rather complicated structure; thus, it is difficult to perform compensation for the wave-front performance, transmission rate and so on, and the design and fabrication of the NA switching element becomes quite cumbersome.